RTOS 任务通知
用作轻量级二进制信号量

相关页面：

• RTOS 任务通知
• 将 RTOS 任务通知用作轻量计数信号量
• 将 RTOS 任务通知用作轻量级事件组
• 使用 RTOS 任务通知作为轻量级邮箱

通过直接通知解除 RTOS 任务阻塞状态的速度和使用二进制信号量相比快 45%， 且前者使用的 RAM 更少。 本页 演示了如何实现这一点。

二进制信号量是一种最大计数为 1 的信号量，因此称为“二进制”。 只有在信号量可用的情况下，任务才能“获取”信号量， 而只有在信号量的计数为 1 的情况下，信号量才可用。

当使用任务通知代替 二进制信号量时，接收任务的 通知值用于代替 二进制信号量的计数值，ulTaskNotifyTake() （或 ulTaskNotifyTakeIndexed()） API 函数用于代替信号量的 xSemaphoreTake() API 函数。 ulTaskNotifyTake() 函数的 xClearOnExit 参数设置为 pdTRUE， 因此每次获取通知时计数值返回为零 — 仿真二进制信号量。

同样，xTaskNotifyGive() （或 xTaskNotifyGiveIndexed()）或 vTaskNotifyGiveFromISR()（或 vTaskNotifyGiveIndexedFromISR() ）函数用于代替信号量的 xSemaphoreGive() 和 xSemaphoreGiveFromISR()h 函数 。

请参阅以下示例。

/* This is an example of a transmit function in a generic
peripheral driver.  An RTOS task calls the transmit function,
then waits in the Blocked state (so not using an CPU time)
until it is notified that the transmission is complete.  The
transmission is performed by a DMA, and the DMA end interrupt
is used to notify the task. */

/* Stores the handle of the task that will be notified when the
transmission is complete. */
static TaskHandle_t xTaskToNotify = NULL;

/* The index within the target task's array of task notifications
to use. */
const UBaseType_t xArrayIndex = 1;

/* The peripheral driver's transmit function. */
void StartTransmission( uint8_t *pcData, size_t xDataLength )
{
    /* At this point xTaskToNotify should be NULL as no transmission
    is in progress.  A mutex can be used to guard access to the
    peripheral if necessary. */
    configASSERT( xTaskToNotify == NULL );

    /* Store the handle of the calling task. */
    xTaskToNotify = xTaskGetCurrentTaskHandle();

    /* Start the transmission - an interrupt is generated when the
    transmission is complete. */
    vStartTransmit( pcData, xDatalength );
}
/*-----------------------------------------------------------*/

/* The transmit end interrupt. */
void vTransmitEndISR( void )
{
BaseType_t xHigherPriorityTaskWoken = pdFALSE;

    /* At this point xTaskToNotify should not be NULL as
    a transmission was in progress. */
    configASSERT( xTaskToNotify != NULL );

    /* Notify the task that the transmission is complete. */
    vTaskNotifyGiveIndexedFromISR( xTaskToNotify,
                                   xArrayIndex,
                                   &xHigherPriorityTaskWoken );

    /* There are no transmissions in progress, so no tasks
    to notify. */
    xTaskToNotify = NULL;

    /* If xHigherPriorityTaskWoken is now set to pdTRUE then a
    context switch should be performed to ensure the interrupt
    returns directly to the highest priority task.  The macro used
    for this purpose is dependent on the port in use and may be
    called portEND_SWITCHING_ISR(). */
    portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
}
/*-----------------------------------------------------------*/

/* The task that initiates the transmission, then enters the
Blocked state (so not consuming any CPU time) to wait for it
to complete. */
void vAFunctionCalledFromATask( uint8_t ucDataToTransmit,
                                size_t xDataLength )
{
uint32_t ulNotificationValue;
const TickType_t xMaxBlockTime = pdMS_TO_TICKS( 200 );

    /* Start the transmission by calling the function shown above. */
    StartTransmission( ucDataToTransmit, xDataLength );

    /* Wait to be notified that the transmission is complete.  Note
    the first parameter is pdTRUE, which has the effect of clearing
    the task's notification value back to 0, making the notification
    value act like a binary (rather than a counting) semaphore.  */
    ulNotificationValue = ulTaskNotifyTakeIndexed( xArrayIndex,
                                                   pdTRUE,
                                                   xMaxBlockTime );

    if( ulNotificationValue == 1 )
    {
        /* The transmission ended as expected. */
    }
    else
    {
        /* The call to ulTaskNotifyTake() timed out. */
    }
}